Method of detecting and locating corrosion sites

ABSTRACT

Method of detecting and locating areas responsible for incipient galvanic corrosion on a metallic surface. Localized electrochemical cells which occur on metallic surfaces as a result of anodic and cathodic sites in contact with an electrolytic solution and which results in corrosion at such locations are detected by immersing the surface in an aqueous electrolytic solution containing a suspension of particles bearing an electrical charge of opposite polarity to the charge carried by the localized site. Particles in the suspension are attracted by the opposite polarity of the localized sites, if any, on the metallic surfaces, causing such particles to accumulate at the locations of the sites. These accumulations of particles remain on the metallic surface following withdrawal from the aqueous solution. Coated metallic surfaces can be examined for the existence of pinholes, cracks, and other penetrating surface discontinuities by subjecting the metallic surface to an electrical charge of polarity opposite that of the particles suspended in solution.

Tlttlle Wellhorn, ,llr,

[ MlETll-llUlD @IF DETECTING AND LOCATTNG CURUSTUN SllTlES [76]Inventor: Hadley W, Wellhorn, .lr., RFD No. 2, Petrie Rd., PowderSprings, Ga. 30073 [22] Filed: Nov. 24, 1972 [21] Appl. No.: 309,500

[52] ILLS. Cl 204N100 R, 204/147, 204/181,

204/195, 204/196, 204/1 T [51] lllmt. Cl 301k 5/00, C23b 13/00 [58]Field of Search 204/180 R, 181, 147,

References (Iited UNITED STATES PATENTS 2,762,767 9/1956 Mosher et a1204/147 2,857,320 10/1958 Hughes 204/1 T 2,869,003 l/l959 Marsh et al204/147 X 2,897,126 7/1959 George 204/180 R X 2,900,320 8/1959 Metcalfeet a]. 204/180 R X 3,168,455 2/1965 Shapiro et a1 204/147 [4 @tet, 16,11973 527,556 mmznr maer John H 0 Assistant Examiner-A. C. PrescottAttorney-Harold D. Jones, Jr. et a1.

[5 7] ABSTRACT tracted by the opposite polarity of the localized sites,

if any, on the metallic surfaces, causing such particles to accumulateat the locations of the sites. These accumulations of particles remainon the metallic surface following withdrawal from the aqueous solution.

Coated metallic surfaces can be examined for the existence of pinholes,cracks, and other penetrating surface discontinuities by subjecting themetallic surface to an electrical charge of polarity opposite that ofthe particles suspended in solution.

0 Claims, No Drawings METlHitOtID Ull DETECTIING AND LO CA'llIlNGCUMRUSHON Sll'lllES This invention relates in general to corrosion andin particular to a method of detecting and locating sites of actual orincipient corrosion on metallic surfaces.

The problems constantly presented by the susceptibility of metals tocorrosion are notorious and need little elaboration here. It iswell-known that large amounts of money and effort are devoted topreventing the occurrence of corrosion on metallic products of manykinds. Not only does unwanted corrosion result in reduction of themetallic surface, leading to weakening of the metal and possiblestructural failure, but the actual corrosion products, which frequentlyoccupy a volume greater than the volume of the corroded metal, can alsoexert a damaging effect on a structural assembly.

Because of the importance attached to the prevention of unwantedcorrosion of metal, it is desirable to be able to detect and locatemetallic surface sites which are responsible for the galvanic corrosionof a metal which is susceptible of corrosion, even though no evidence ofactual corrosion may be apparent from a visual inspection of such sites.These sites, which may be identified as locations of incipientcorrosion, are generally considered to occur as a result of theformation of localized electrochemical cells involving the detectablegalvanic locations on a metallic surface. it is believed in the art thatthese cells are formed between different phases in the metal, betweenimpurities and inhomogeneities and the bulk of the metal, betweenprecipitates at grain boundaries and the grains themselves, or by othersimilar conditions wherein electrically connected surface areas havingdifferent electromotive potentials are available to come in contact witha corrosive environment. It will be understood that the term corrosiveenvironment broadly includes any environment which permits anoxidation-reduction reaction to occur, and that mere exposure to normalatmospheric conditions is a corrosive environment of a degree.

The localized electrochemical cells which constitute areas of incipientcorrosion are considered to consist of anodic areas where solution ofthe metallic surface occurs, causing formation of corrosion pits, andcathodic areas where there is a corresponding chemical reduction. Priorart techniques have been proposed for locating cathodic and anodiccorrosion sites on metallic surfaces. One example of the prior art isfound in Corrosion, Volume 18, pages 2391 246:, June 1962. Thistechnique involves the use of complex and expensive equipment, includingan oscilloscope, to synthesize a map of the distribution and intensityof corrosion potential on a metallic surface. it is apparent that theneed exists for a technique which will rapidly and inexpensively enablelocations of corrosion and incipient corrosion on'metallic surfaces tobe detected and located.

Accordingly, it is an object of the present invention to provide animproved method for detecting incipient galvanic corrosion.

llt is another object of the present invention to provide an improvedmethod of detecting and locating sites which lead to the formation ofgalvanic corrosion cells.

It is still another object of the present invention to provide animproved method for detecting the existence and the location ofpenetrating discontinuities in coatings on a metallic surface.

Other objects and many of the attendant advantages of the presentinvention will become more apparent from the following description ofthe preferred embodiment disclosed herein.

Stated in general terms, the method of the present invention comprisessubjecting a metallic surface to an aqueous electrolytic solutioncontaining a suspension of particles which bear an electrical charge ofpolarity opposite to the polarity of the corrosion-forming localizedelectrochemical sites on the metal surface. The charged particles areelectrically attracted to such localized areas because of oppositepolarities of charges, and the localized electrochemical sites are thusidentifiable by the accumulations of particles remaining on the metallicsurface when withdrawn from the aqueous solution. Similarly, a metallicsurface coated with a dielectric coating can be examined for pinholes orother discontinuities in the surface by first charging the metallicsurface at a polarity which is the opposite of the particles in theaqueous suspension, and then subjecting the coated surface to thesuspension of charged particles.

The nature and practice of the present invention will be more apparentwith reference to the following description of a specific disclosedembodiment thereof. An aqueous solution prepared containing a suspensionof fused SiO (silica) particles in water. The average particle size isabout six microns in diameter, and the suspension contains approximately82 percent by weight of the silica particles. The pH of the suspensionis generally between 4.0 and 5.0, as a result of the formation ofsilicic acid during its composition, although the pH can be adjusted toany desired value within the range of about 2.0-7.0 with acid or baseadditions. The viscosity of the exemplary suspension is aboutcentipoise.

Samples of metal such as iron, steel, brass, aluminum, and the like werecleaned to remove grease and surface film and were then immersed in theforegoing aqueous electrolytic suspension. The metal samples acquired athin coating of the suspension by simple wetting, but after immersion ismaintained for a few minutes, a number of depositions of the silicaparticles became visibly apparent at various sites on the surface of themetal. These depositions appear as small mounds or bumps of the silicaparticles and are readily observable after an immersion time of only afew minutes. When the metallic sample is removed from the suspension,the nonadherent suspension wetting the entire sample is briskly shakenoff or very carefully rinsed off with water.'The adherent depositionscan be conveniently scraped away, if desired, while the sample is beingviewed through a microscope or other suitable apparatus. Microscopicexamination of the metal surface sometimes reveals that the depositionshave formed on pre-existing corrosion pits or sometimes on what appearto be inclusions or areas of inhomogeneity in the metal.

The method of the present invention can be used for analytical andinvestigative purposes apart from mere detection of incipient corrosionsites. As an example, the ionic content of the aqueous solution can beadjusted to simulate various types of actual corrosive environments andthe effect of such adjustment can be evaluated by noting thecorresponding effect on the deposition of particles suspended in theaqueous solution. Such adjustment of the ionic content could consist ofthe addition of sea salts in an amount to simulate a sea waterenvironment, for example, or the addition of ions known to increase orto inhibit the rate of corrosion.

As an experiment confirming the foregoing theoretical operation of thepresent invention, a pair of electrodes made of metal or graphite areconnected to an external source of EMF and disposed in the suspension. Avery rapid deposition of suspension particles occurs on the electrodeconnected to the positive potential source, and this depositioncontinues to the extent of removing most of the particles from thesuspension within a few minutes. Accordingly, it appears that thesuspension particles are negatively charged and, by inference, thedeposition sites on metallic surfaces are, or include, anodic poles.

The method of the present invention is applicable to investigation ofcoated metallic surfaces to determine the existance and location of anypinholes, scratches, or other surface discontinuities which penetratethe thickness of the coating. The coated metal sample is made to be thepositive electrode by appropriate connection to an external source ofEMF, and the sample is then immersed in a suspension as described above.Any coating discontinuities which expose the underlying charged metallicsurface to the suspension particles will become apparent by depositionof particles upon the discontinuities. Alternatively, if nodiscontinuities exist in the coating, this fact is made apparent by theabsence of any localized deposition of particles on the coated surface.

Although the method of the present invention is practicable with anysuspension of particles which bear the proper electrical charge withrespect to the polarity of the corrosion sites being investigated, theuse of aqueous suspensions of amorphous (fused) silica are found toprovide superior results and are preferred. The particle size andparticle concentration is not considered to be critical, provided thatthe particle size is such as to maintain the particles in suspension forthe duration of a corrosion site test according to the invention.

Furthermore, it will be understood that the foregoing relates only to apreferred embodiment of the present invention, and that numerousalterations and modifications may be made therein without departing fromthe spirit and scope of the invention.

What is claimed is:

1. Method of detecting sites of potential corrosion on a metallicworkpiece, comprising the steps of subjecting a metallic surface to anaqueous electrolytic suspension containing electrically chargedparticles the charge of which are opposite to that of suspected sites ofpotential corrosion on the metallic surface, observing the locations onthe surface of any accumlations of said particles adhering thereto, anddetermining from said locations where the sites of potential corrosionare located.

2. The method of claim 1, wherein said metallic surface is subjected toan aqueous electrolytic suspension of particles of amorphous (fused)silica.

3. The method of claim 1, wherein said step of subjecting comprisesimmersing the surface in said aque-' ous electrolytic suspension,subsequently removing the surface from the aqueous suspension, andobserving the location on the surface of any accumulation of saidparticles adhering thereto.

4. The method of claim 3, further comprising the step of removing theresidual quantity of said suspension which remains on the surface afterremoval from the aqueous electrolytic suspension.

5. The method of claim 1, for detecting penetrating discontinuities inthe coating of a coated metallic surface, additionally comprising thesteps of charging the metallic surface with an electromotive force ofpolarity opposite the polarity of said charged particles in suspension.

6. The method of claim 5, wherein said step of subjecting comprisesimmersing the coated charged surface in said aqueous suspension,subsequently removing the coated charged surface from the aqueoussuspension, and marking the location on the coated surface of anyaccumulation of said particles adhering thereto.

7. The method of claim 6, further comprising the step of removing theresidual quantity of said aqueous suspension which remains on the coatedsurface after removal from said solution.

8. The method of claim 1, wherein the corrosion sites being detectedhave an electrically positive charge, and wherein said aqueous solutioncontains a suspension of particles having an electrically negativecharge.

2. The method of claim 1, wherein said metallic surface is subjected toan aqueous electrolytic suspension of particles of amorphous (fused)silica.
 3. The method of claim 1, wherein said step of subjectingcomprises immersing the surface in said aqueous electrolytic suspension,subsequently removing the surface from the aqueous suspension, andobserving the location on the surface of any accumulation of saidparticles adhering tHereto.
 4. The method of claim 3, further comprisingthe step of removing the residual quantity of said suspension whichremains on the surface after removal from the aqueous electrolyticsuspension.
 5. The method of claim 1, for detecting penetratingdiscontinuities in the coating of a coated metallic surface,additionally comprising the steps of charging the metallic surface withan electromotive force of polarity opposite the polarity of said chargedparticles in suspension.
 6. The method of claim 5, wherein said step ofsubjecting comprises immersing the coated charged surface in saidaqueous suspension, subsequently removing the coated charged surfacefrom the aqueous suspension, and marking the location on the coatedsurface of any accumulation of said particles adhering thereto.
 7. Themethod of claim 6, further comprising the step of removing the residualquantity of said aqueous suspension which remains on the coated surfaceafter removal from said solution.
 8. The method of claim 1, wherein thecorrosion sites being detected have an electrically positive charge, andwherein said aqueous solution contains a suspension of particles havingan electrically negative charge.